The present invention relates to a hardened Fe-Ni alloy and to a process for manufacturing it in the form of thin strip that can be especially used for the manufacture of integrated circuit leadframes or of electron gun grids for color display cathode-ray tubes.
In electronic components comprising integrated circuits fabricated from silicon chips, the silicon chips are cemented to leadframes intended especially to form the connection leads of the integrated circuits. These leadframes must be made of an alloy having many properties making them compatible with the silicon chips and allowing satisfactory fabrication of the components. In particular, the alloy must have a thermal expansion coefficient compatible with that of silicon so as to prevent the generation of high mechanical stresses during the thermal cycles involved in the fabrication operations. The alloy must have a mechanical strength sufficient for the connection leads not to deform during handling by means of automatic machines, but it must be sufficiently ductile to allow these leads to be formed by bending. The alloy must also be able to be easily cut, either by chemical cutting or by mechanical cutting, and its surface must not be easily oxidizable in order to allow electroplating to be carried out properly.
Usually, integrated circuit leadframes are made of an N42(copyright) alloy which is an Fexe2x80x94Ni alloy containing about 41% Ni. However, this alloy has the drawback of not having a mechanical strength high enough to produce thin integrated circuit leadframes with a large number of connection leads. This difficulty is especially apparent in the case of integrated circuits called xe2x80x9cQuad Flat Packsxe2x80x9d, the number of connection leads of which may reach 240, or integrated circuits called
xe2x80x9cThin Small Outline Packsxe2x80x9d, the total thickness of which is 1 mm.
To remedy this drawback and to manufacture thin integrated circuits having a large number of connection leads, it has been proposed to use various hardened Fexe2x80x94Ni alloys, which all have drawbacks.
For example:
in Japanese patent applications J 04160112, J 04072037 and J 04099252, it has been proposed to use Fexe2x80x94Ni alloys hardened by solid solution and work-hardening consolidation, but these alloys have a low ductility making them ill-suited for forming by bending;
in Japanese patent applications J 03207835, J 03207834 and J 03173740, it has been proposed to use Fexe2x80x94Ni alloys hardened by homogeneous precipitation of either xcex3xe2x80x2 or xcex3xe2x80x3 NiBe phases by the addition of elements such as Be, Ti, Al, Nb or Ta, but these alloys have the drawback of having a reduced cutting speed and a greater sensitivity to surface oxidation compared with those of alloy N42;
in patent U.S. Pat. No. 5,026,345, it has been proposed to use an Fexe2x80x94Ni alloy hardened by work-hardening martensite formation, but this alloy has the drawback of having a low ductility and a much higher expansion coefficient than that of N42;
in patent application EP 0 489 932, it has been proposed to use an F-Ni alloy hardened by precipitation of Ti, Zr, Hf, V, Nb or Ta carbides and by work-hardening consolidation, but this alloy has an increased sensitivity to surface oxidation, thereby degrading the ability to be electroplated.
None of the alloys that have just been mentioned has all the desired properties and it is an object of the present invention to remedy this drawback by providing an alloy with a high yield strength which is better suited to the manufacture of integrated circuit leadframes than the known alloys.
For this purpose, the subject of the invention is a process for manufacturing an Fexe2x80x94Ni alloy whose chemical composition comprises, by weight:
36%xe2x89xa6Ni+Co xe2x89xa643%
0%xe2x89xa6Coxe2x89xa63%
0.05%xe2x89xa6Cxe2x89xa60.4%
0.2%xe2x89xa6Crxe2x89xa61.5%
0.4%xe2x89xa6Moxe2x89xa63%
0%xe2x89xa6Cuxe2x89xa63%
Sixe2x89xa60.3%
Mnxe2x89xa60.3%,
the balance being iron and impurities, this alloy having a yield strength Rp0.2 greater than 750 MPa and a uniform elongation Ar greater than 5%. Preferably, the yield strength Rp0.2 is between 800 MPa and 1100 MPa and the tensile strength Rm is between 900 MPa and 1130 MPa.
Preferably, the chemical composition is such that:
38.5%xe2x89xa6Ni+Coxe2x89xa641%
0.1%xe2x89xa6Cxe2x89xa60.35%
0.5%xe2x89xa6Crxe2x89xa61.2%
1.5%xe2x89xa6Moxe2x89xa62.5%.
Also preferably, the uniform elongation Ar is greater than 7%, the yield strength Rp0.2 is between 800 MPa and 1000 MPa and the tensile strength Rm is between 900 MPa and 1040 MPa.
According to the process for manufacturing the strip:
an alloy whose chemical composition is in accordance with the composition defined above, is smelted and, optionally, vacuum remelted or electroslag remelted in order to obtain a semifinished product;
the semifinished product is hot rolled at a temperature greater than or equal to 950xc2x0 C. in order to obtain a hot strip having a thickness of between 2 and 6 mm, and preferably between 3 and 5 mm, the hot rolling optionally being preceded by a homogenization treatment by a soak above 950xc2x0 C.; after rolling, the strip is cooled to below 450xc2x0 C. at a cooling rate sufficient to avoid the precipitation of carbides;
the hot strip is cold rolled in one or more 5% to 95% thickness reductions separated by anneals at a temperature greater than 950xc2x0 C.; and
a hardening heat treatment is carried out between 450xc2x0 C. and 850xc2x0 C., the hardening heat treatment being preceded by an at least 40% thickness reduction.
Optionally, after the hardening heat treatment, a supplementary cold-rolling operation is carried out followed by a recovery heat treatment between 550xc2x0 C. and 750xc2x0 C.
The invention also relates to strip made of Fexe2x80x94Ni alloy according to the invention and especially a strip whose thickness is greater than or equal to 0.1 mm and the uniform elongation Ar is greater than or equal to 5%. This strip can be used for the manufacture of an integrated circuit leadframe or for the manufacture of an electron gun grid. It is thus possible to obtain an integrated circuit leadframe made of an alloy whose thermal expansion coefficient between 20xc2x0 C. and 300xc2x0 C. is preferably between 4xc3x971031 6/K and 6xc3x9710xe2x88x926/K.